The invention relates to optical waveguide gratings and more particularly to optical waveguide gratings having a step or discontinuity in the periodicity of the grating.
As pointed out by H. A. Haus and C. V. Shank in the article entitled "Antisymmetric Taper of Distributed Feedback lasers" , IEEE Journal of Quantum Electronics, Vol. QE--12, No. 9, Dec. 19, 1976, pages 532-539, a distributed feedback laser with no mode degeneracy can be created by using an optical waveguide grating having a quarter wavelength step or discontinuity in the middle of the grating. This type of distributed feedback laser is also described in their copending patent application entitled "Distributed Feedback Devices with Perturbations Deviating from Uniformity for Removing Mode Degeneracy", filed Feb. 7, 1976, Ser. No. 654,339. As pointed out by Messrs. Haus and Shank, this type of distributed feedback laser is preferable in that the output wavelength is entirely predictable unlike the normal distributed feedback laser having a grating with regular periodicity throughout.
One technique presently known to the art for creating an optical waveguide grating is described by G. C. Bjorklund, L, F. Mullenauer, and W. J. Tomlinson in their copending application entitled "Apparatus for Piecewise Generation of Grating-Like Pattern", filed Jan. 12, 1976, Ser. No. 648, 326. This apparatus is also described in their article entitled "Piecewise Interferometric Generation of Precision Gratings", by L. F. Mullenauer and W. J. Tomlinson, Applied Optics, Vol. 16, No. 3, March 1977, pages 555-557. In the Bjorklund et al apparatus a grating can be generated in a piecewise fashion by moving the interference pattern across an optical substrate having a photoresist layer. To adopt this technique so as to produce a step or discontinuity in the grating would require the mechanical insertion of a 180.degree. phase shifter into the path of one of the coherent beams used to create the grating. This could only be done after the interference pattern has been moved so as to not overlap a previously created pattern. The precise movement of the two coherent beams and mechanical insertion of the phase shifter are believed to be formidable problems in the creation of an optical waveguide grating having a quarter wavelength step in the structure with minimum separation between the two segments of the grating.